The present invention relates to the field of optical devices such as radiation sources and detectors. More specifically, the present invention relates to optical devices used to efficiently generate a beam of radiation with a given frequency or within a band of frequencies. The present invention may also extend to an imaging system.
Difficulties arise in producing electromagnetic radiation in several commercially important wavelength bands. For instance, in the terahertz (THz) frequency range (100 Ghz to 20 THz), near and mid-infra red (wavelength=1000-5000 nm) and blue/UV (wavelength=400-450 nm).
A possible method of producing such frequencies is to use non linear optical effects. The polarisation P in the material induced by the incident radiation can be expressed in terms of E the electric field exciting the material as the power series:
P="khgr"1E+"khgr"2E2+"khgr"3E3 . . . 
Generally, the relationship Pxcex1E is used and the higher order terms are assumed to be negligibly small. This approximation does not hold for large E. Non linear optics is concerned with these higher order terms.
If a material is irradiated with two different frequencies, the second order term allows the material to emit frequencies which are the sum of the input frequencies (known as Sum Frequency Generation), the difference between the input frequencies (Difference Frequency Generation) The second order susceptibility can also result in the generation of different optical frequencies when the material is irradiated by a single input frequency. For instance, second harmonic generation results from self sum generation. For optical parametric conversion, two frequencies are generated from the input frequency.
Also, the third order term "khgr"3E3 can also be excited to produce third harmonic frequencies and other third order terms.
The problem which arises in such structures is that to obtain emitted radiation with the desired frequency efficiently, the phase of the polarisation induced by the incident radiation needs to be matched as closely as possible to the phase of the desired emitted radiation.
This phase matching problem also arises in certain types of detectors where changes in a reference beam with a first frequency due to a beam with a second frequency are used to measure the beam with the second frequency.
Some materials have a natural degree of phase matching due to birefringence properties and hence phase matching can be achieved over at least a certain length of the material. However, many materials with large optical non linearities nevertheless suffer from having no birefringence or other properties which allow some degree of phase matching and thus the full realisation of the material as a frequency converter.
The present invention addresses the above problems and in a first aspect, provides an optical device comprising an optical modulation region which comprises phase matching means for enhancing the phase matching between at least two different frequency signals propagating in the optical modulation region in response to illumination by at least one incident beam of radiation, the phase matching means having a spatial variation in its refractive index along a component of the incident radiation beam configured to maximise a distance in the modulation region over which the at least two different frequency signals stay in phase.
To clarify the above, a component of the incident radiation beam is a directional component of the beam.
If the present invention is used for frequency conversion, the optical modulation region preferably comprises frequency conversion means for emitting a beam with an emitted radiation frequency in response to illumination with the at least one incident beam of radiation, the phase matching means enhancing the phase matching between the polarisation generated by the or an incident beam and the emitted beam. The emitted beam having a different frequency to that of the incident beam.
There are many possible mechanisms for converting the frequency of the input radiation.
A new frequency can be generated from incident radiation with two or more frequencies, by so called sum or difference frequency generation.
The optical device is illuminated with radiation which either has two frequency components or the device is illuminated with two beams of radiation of different frequencies.
The higher fields obtainable by pulsed lasers allow the (progressively smaller) non linearities in the polarisation term to be accessed.
Preferably, a pulsed laser is used in order to generate the two incident frequencies. Also, the radiation pulse, produced by a pulsed laser inherently contains a number of different frequencies making it ideal for sum or difference frequency generation over a broad range.
However, with the enhanced phase matching achievable with the present invention, lower fields can be used and hence the two frequencies could be provided by two CW lasers operating at different frequencies. These provide a continuous beam which does not provide as high an electric field as a pulsed system although non-linear effects still may be accessed with CW lasers. CW lasers also have the advantage that they are presently more ubiquitous and commercially available than pulsed lasers.
The incident radiation generates a time-dependent polarisation via the second order non linearity of the material. A simplified view of the mechanism is to picture the electrons in the material as being on springs. The incident radiation causes the electrons to vibrate with frequencies corresponding to the incident frequencies, their sum and their difference.
Which frequency is emitted is dependent on the non-linear coefficients of the material at the fundamental frequency, and phase matching between the non-linear polarisation and generated/converted radiation at the sum/difference frequencies.
For sum frequency generation, the phase matching condition is given by
xcex94k=k(xcfx891)+k(xcfx892)xe2x88x92k(xcfx893)=0,
where xcfx891+xcfx892=xcfx893 and k(xcfx89) is the k-vector of the light in the material at frequency xcfx89. For difference frequency generation the phase matching condition is given by
xcex94k=k(xcfx891)xe2x88x92k(xcfx892)xe2x88x92k(xcfx893)=0
where xcfx891xe2x88x92xcfx892=xcfx893.
For Nth harmonic generation the phase matching condition is given by
xcex94k=Nk(xcfx891)xe2x88x92k(xcfx892)=0
where N xcfx891=xcfx892.
For optical parametric generation the phase matching condition is given by
xcex94k=k(xcfx891)xe2x88x92k(xcfx892)xe2x88x92k(xcfx893)=0
where xcfx891=xcfx892+xcfx893.
The coherence length, which can be thought of as the distance over which the generated field becomes out of phase with the driving field, is defined as
lc=2/xcex94k
The refractive index, n, of the material is defined by n=kc/xcfx89, where c is the speed of light in vacuum. Since the refractive index of many non-linear materials varies with the frequency xcfx89, the coherence length is typically just a few microns. The present invention modifies the dispersion, i.e. the variation of n with xcfx89, in order to satisfy the phase matching condition xcex94k=0.
Considering the case where THz radiation with a frequency xcfx89THz is generated from two visible radiation frequencies xcfx89oph using the technique of difference frequency generation. Phase matching for the generating THz radiation from the difference between the frequencies of the incident radiation is:
xe2x80x83xcex94k=k(xcfx89opt+xcfx89THz)xe2x88x92k(xcfx89opt)xe2x88x92k(xcfx89THz)=0.
The coherence length, lc, which is a measure of the distance over which the optically-induced non-linear polarisation and the generated THz electric fields remain in phase, is defined:
lc=xcfx80c/(xcfx89THz|xcex7visxe2x88x92xcex7THz|)
where xcex7vis is the refractive index of the material at visible frequencies and xcex7THz is the refractive index of the material at THz frequencies,
Thus, for the long coherence length necessary for efficient THz generation, xcex7visxe2x88x92xcex7THz must be small. However, inorganic non-linear optical materials have a large index mismatch and so lc may be no larger than a few microns.
The present invention enhances the phase matching by reducing this variation in the refractive indices at the different frequencies.
Similarly, the modulation region can be configured such that the second order coefficient of the addition of the two frequencies is more efficiently emitted. Alternatively, the second harmonic can be selected instead.
A further possible mode of operation is by optical parametric generation. The optical device is pumped by a single laser.
Due to the non-linear character of the material, two beams with different frequencies can be emitted or the second harmonic can be emitted.
The two different frequencies are related to the input frequency by momentum and energy conservation.
The above devices can be supplied by radiation from one or two diode lasers. Therefore, the laser and the optical device can be integrated together on a single chip Also, a pulse laser can be integrated on a single chip with the frequency converter.
The above phase matching principles can also be applied to detectors, in this case, the modulation region is preferably configured to rotate the polarisation vector of a first input beam in response to illumination with a second input beam and emit a beam with the rotated polarisation vector,
The first input beam can be thought of as a reference beam, the second beam is the detected beam. Before entering the detector, the polarisation of the reference beam and the detected beam are rotated such that they are parallel to one another and have a component along both the ordinary and extraordinary axes of the modulation region.
The reference beam and the detected beam are preferably linearly polarised before entering the optical modulator. However, one or both of them may also be circularly polarised.
The modulation region is configured so that the detected beam (if present) rotates the polarisation of the reference beam by an angle. If the detected beam is not present, the reference beam may become slightly elliptically polarised during its passage through the optical modulation region. To compensate for this effect, the beam emitted from the modulation region is preferably passed through an optical correction circuit.
Preferably, the optical correction circuit converts the linearly (or elliptically) polarised beam into a circularly polarised beam,
The emitted linearly (or elliptically) polarised beam is preferably converted into a circularly polarised beam by a quarter wave plate. Preferably, this circularly polarised beam is then passed into a polarisation splitting device (such as a Wollaston prism) which separates the circularly polarised radiation into two linear components. If the detected beam is not present, these linear components are equal. If the detected beam is present, these linear components are not equal to one another.
Preferably, the two output beams from the prism are incident on a balanced photodiode assembly, which produces a non-zero output signal if there is a difference in magnitude between the two beams.
Alternatively, the reference beam can be circularly polarised before entering the detector. This can be achieved by putting the quarter wave plate in the path of the reference beam before it reaches the detector. The detected beam (if present) changes the circularly polarised reference beam into an elliptically polarised beam. The elliptically polarised beam is then separated into its two components by a Wollaston prism or the like as described previously.
The optical device of the present invention can therefore be used in both the generation of radiation and the detection of radiation. Hence, an imaging system is provided which uses the present invention configured as a generator (frequency converter) to provide the imaging radiation and the present invention configured as a detector to detect the imaging radiation. It will be appreciated by a man skilled in the art that an imaging system can have either of the generator or the detector as described above coupled with a conversion detector or generator respectively.
Preferably a beam is split to form an input for the generator and a reference beam for the detector. Imaging radiation is produced by the generator to image a sample. The radiation carrying the image information is then detected by the detector using the reference beam. Preferably a control system is provided which provides a time variation between the input beam and the reference beam.
Possible materials which posses good non linear characteristics for any of the above mechanisms are GaAs or Si based semiconductors. More preferably, a crystalline structure is used. The following are further examples of possible materials: NH4H2PO4, ADP, KH2PO4, KH2ASO4, Quartz, AlPO4, ZnO, CdS, GaP, BaTiO3, LiTaO3, LiNbO3, Te, Se, ZnTe, ZnSe, Ba2NaNb5O15, AgAsS3, proustite, CdSe, CdGeAs2, AgGaSe2, AgSbS3, ZnS, organic crystals such as DAST (4-N-methylstilbazolium).
In general, non-centrosymmetric crystals are used for second order effects. Third order effects are found in a variety of different crystals with varying strengths.
It is preferable if such a structure is placed in a waveguide configuration which operates at least one of the input beam frequencies. The waveguide can be realised by either a single cladding layer and a core layer or a core layer interposed between two cladding layers. For example, a GaAs layer interposed between AlGaAs cladding layers.
Ideally, the material has a periodic variation in its refractive index. Preferably, the refractive index will vary with a pitch of between 100 nm and 10 xcexcm. More preferably between 0.5 xcexcm and 5 xcexcm.
The variation in the refractive index can be achieved in a number of ways. A simple method is to just etch holes into the non-linear optical material such that a local variation in the refractive index is caused within the path of the beam.
The holes can be formed in just the core layer, the core layer and one or both of the cladding layers or one or both of the cladding layers with no holes formed in the core layer. More complex structures could use two materials with different refractive indices. For example GaAs and AlGaAs. A first material with a first refractive index has holes or slots provided in it and the second material which has a different refractive index to the first material fills the holes or the slots of the first material.
The variation in the refractive index produces a variation in the component k-vector (momentum direction) along the direction in which the index varies or is modulated.
However, in practice, the beam might be perpendicular to an input surface of the modulation region such that the k-vector of the beam is parallel to the direction in which the refractive index varies for maximum effect. Alternatively, the beam may be incident at an angle to the input surface, say, for example, at 45xc2x0. In this case, only a component of the input beam""s k-vector is parallel to the direction in which the variation in refractive index occurs,
Such structures can be achieved by regrowth, ion beam implantation, ion beam lithography and standard lithographic patterning techniques. Preferably, such structures are provided with straight boundaries between the regions with different refractive indices.
It may be preferable in some cases to integrate a source for supplying the incident beam or beams with the optical modulation region. The source may be a single CW source (for parametric generation or second harmonic generation) or a pulsed laser source or two CW sources for sum and difference frequency generation.
The optical modulation region may be located within a mirror cavity. Further, the at least one incident beam of radiation may be produced within a lasing cavity and said optical modulation region may be provided within the said cavity.
In a second aspect, the present invention provides a radiation source comprising.
a frequency conversion member for emitting a beam of radiation in response to irradiation by one or more input beams, the emitted beam having a frequency different to that of the one or more input beams,
the one or more input beams all being produced within a lasing cavity and said frequency conversion member being located within said lasing cavity.
The above said frequency conversion member has frequency conversion means are hereinbefore described with reference to the optical modulation region. Preferably, the frequency conversion member is configured to emit a beam with a frequency substantially equal to the difference in frequency between two input beams. More preferably, the frequency conversion member is configured to emit radiation in the THz frequency range (i.e. 100 GHz to 20 THz).
The lasing cavity may be the lasing cavity of a pulse laser. It may also be the lasing cavity of one or more CW laser sources, e.g. solid state diodes.
The second aspect of the present invention is particularly of use in the field of difference frequency generation, e.g where two beams at visible frequencies xcfx89vis1 and xcfx89vis2 are converted to THz radiation at xcfx89THz via non-linear difference frequency generation xcfx89THz=xcfx89vis1xe2x88x92xcfx89vis2.
Generating THz radiation using difference frequency generation suffers from the problem that in order to produce THz radiation with a commercially useful power level, either a large power density of input radiation is required or the THz signal must be amplified.
The conversion efficiency xcfx81 from xcfx89vis1 and xcfx89vis2 to xcfx89THz is given by:   ρ  =                    P        ⁢                  (                      ω            THz                    )                            P        ⁢                  (                      ω            vis                    )                      ≈          2      ⁢                        (                      μ                          ϵ              0                                )                          3          2                    ⁢                                    ω            vis            2                    ⁢                      d            2                    ⁢                      l            2                    ⁢                                    sin              2                        ⁢                          (                              Δ                ⁢                                  xe2x80x83                                ⁢                k                ⁢                                  l                  2                                            )                                                            (                                          η                3                            ⁢                              (                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                  k                  ⁢                                      l                    2                                                  )                                      )                    2                    ⁢                        P          ⁢                      (                          ω              vis                        )                          A            
Where A=area of the beam, d=second order non-linear optical coefficient, xcex7 is the refractive index at xcfx89vis, l is the length of the non-linear crystal,
and where
xcfx89vis=xcfx89vis1≈xcfx89vis2
xcex94k=k(xcfx89vis2)xe2x88x92k(xcfx89vis1)xe2x88x92k(xcfx89THz)(expresses momentum conservation).
It is difficult and expensive to provide an input beam at xcfx89vis with a very high power density outside of a conventional laser cavity. Using amplifiers to amplify either the input radiation or the emitted THz signal increases the bulkiness and the cost of the source. It is precisely this increased size and cost which currently limits the proliferation of THz imaging systems in potential commercial applications.
In the present invention, the frequency conversion member is actually placed within the laser cavity of the input beam or beans, and thus it is termed an intra-cavity structure, Thus, high powers of the input beam are accessed inside the laser cavity, resulting in larger THz powers, without having to resort to bulky or expensive amplifiers placed external to the cavity to realise high input beam powers.
In the following description, the input beam or beams will be referred to as having a frequency of xcfx89vis1, xcfx89vis2 etc., or more generally xcfx89vis as, in general, xcfx89vis1≈xcfx89vis2. The emitted beam will be referred to as having a frequency of xcfx89THz. However, it will be appreciated by a person skilled in the art that although it is preferred that a THz signal is emitted, radiation of other wavelengths could be produced using the radiation source of the present invention In particular, radiation could include mid-infrared (20 THz-90 THz), near-infrared (90 THz-300 THz), and millimeter wave/microwave frequencies (100 GHz-10 GHz). A wide bandwidth signal with powerful components at frequencies ranging from the millimeter wave to near-infrared is achievable with this invention, and this would have wide ranging applications in imaging and spectroscopy.
The intensity of the input beam at xcfx89vis inside a laser cavity exceeds its value outside the lasing cavity by (1xe2x88x92R)xe2x88x921, where R is the lowest reflectivity of the mirrors (at xcfx89vis) within the cavity for the input beam itself If R=1, the intensity enhancement inside the cavity is very large and hence any THz power at xcfx89THz generated by placing a frequency conversion member inside the cavity is also greatly enhanced.
The apparatus can be configured such that one can extract the total available power of the laser at xcfx89THz instead of at xcfx89vis and obtain 100% conversion efficiency However, the frequency conversion member located within the lasing cavity inevitably results in additional losses within the cavity at xcfx89vis. This results in a reduction of the power density (P(xcfx89vis)) in the cavity.
One of the mirrors in the input beam (laser) resonator is referred to as an output coupler and reflects a majority of the input beam back into the laser cavity, allowing (with other components) for lasing action in the cavity at xcfx89vis. The reflectivity of the output coupler is changed (increased) to cancel any losses of power at xcfx89vis inside the resonator which arise when the frequency conversion member is inserted. If no input beam power at xcfx89vis is required, the reflectivity R of the output coupler is ideally set to 100% so that all the power at xcfx89vis remains in the cavity and contributes to the THz generation from the frequency conversion member. However, many practical imaging and spectroscopy systems will require output from the system at both xcfx89vis as well as xcfx89THz, and in this case R is preferably set between 90-100% at xcfx89vis.
The output coupling means may be provided on the opposing side of the frequency conversion member to the side which the input beam or beams first enter the frequency conversion member.
The drop in power occurs at the laser output of xcfx89vis external to the cavity. This is not of direct concern in making a more powerful THz source.
The output coupler is preferably provided by a member which has substantially zero reflectivity to the emitted radiation and highly reflective to the radiation of the input beam or beams. Preferably, this high reflectivity is between 90% and 100%. Thus, the output coupler allows the THz radiation to exit from the cavity, but confines the input beam radiation to within the optical cavity to generate further THz radiation from the frequency conversion member.
The output coupler can be configured to allow a source according to the present invention to emit both THz radiation and input beam radiation. (The advantages of this will be described with reference to imaging systems later in this description.)
Preferably, the output coupler is arranged so that the THz radiation is emitted from the cavity before it is reflected back onto the lasing element which generates the input beam or beams.
To produce THz radiation, the frequency conversion member will preferably be a non linear crystal which is preferably configured to emit a frequency which is substantially equal to the difference of two frequencies provided by the input beam or beams.
The incident radiation generates a time dependent polarisation via the second order non linearity of the material. A simplified view of the mechanism is to picture the electrons in the material as being on springs. The incident radiation causes the electrons to vibrate with frequencies corresponding to the incident frequencies, their sum and their difference. Vibration occurs at sum and difference frequencies due to the non linear nature of the spring vibration.
Which frequency is emitted is dependent on the non-linear coefficients of the material at the fundamental frequency, and phase matching between the non-linear polarisation and generated/converted radiation at the difference frequencies.
The efficiency of the THz generation from two visible photons xcfx89vis1 and xcfx89vis2 is governed by two key material properties, which are summarised below:
1. The Second Order Susceptibility, "khgr"(2) 
The magnitude of "khgr"(2) determines the conversion strength of the visible electric field to THz electric field, and is related to the degree of asymmetry of the electric potential at the microscopic level. This is evident from the fact that the THz power generated is proportional to the polarisation of the material xcfx81(xcfx89THz) oscillating at xcfx89THz given by:
xe2x80x83xcfx81(xcfx89THz)xe2x88x9d"khgr"(2)Evis1Evis2
Crystals which have a large "khgr"(2) which are suitable for the frequency conversion member are:
LiIO3, NH4H2PO4, ADP, KH2PO4, KH2AsO4, Quartz, AlPO4, ZnO, CdS, GaP, GaAs, BaTiO3, LiTaO3, LiNbO3+, Te, Se, ZnTe, ZnSe, Ba2NaNb5O15, AgAsS3, proustitie, CdSe, CdGeAs2, AgGaSe2, AgSbS3, ZnS, BBO, KTP and organic crystals such as DAST (4-N-methylstilbazolium).
2. The Refractive Indices for the Visible and THz Fields (xcex7vis and xcex7THz)
These govern the degree of phase matching between the optically induced non-linear polarisation and the generated THz, which need to interfere constructively throughout the crystal. Materials with a large difference between xcex7vis and xcex7THz suffer from poor conversion efficiencies. Phase matching ensures that the momentum k is conserved between the visible photons with momentum at or near k(xcfx89vis), and the THz photons with momentum k(xcfx89THz) which are generated by the frequency conversion member. Phase matching for generating THz radiation from the difference frequencies of the incident radiation is expressed as:
xcex94k=k(xcfx89vis+xcfx89THz)xe2x88x92k(xcfx89vis)xe2x88x92k(xcfx89THz)≈0.
The coherence length, lc, which is a measure of the distance over which the optically-induced non-linear polarisation and the generated THz electric fields remain in phase, is defined:
lc=xcfx80c/(xcfx89THz|xcex7visxe2x88x92xcex7THz|)
where xcex7vis is the refractive index of the material at visible frequencies and xcex7THz is the refractive index of the material at THz frequencies.
Thus, for the long coherence length necessary for efficient THz generation, xcex7visxe2x88x92xcex7THz must be small. However, inorganic non-linear optical materials have a large index mismatch and so lc may be no larger than a few microns.
Therefore, it is preferable if the frequency conversion means of the second aspect of the invention also comprise phase matching means for enhancing the phase matching between at least two different frequency signals propagating in the frequency conversion member in response to illumination by the one or more input beams, the phase matching means having a spatial variation in its refractive index along a component of the input radiation beams.
Some materials have a natural degree of phase matching due to birefringence properties. In this case, phase matching can be achieved over at least a certain length of the material. However, many materials with large optical non-linearities nevertheless suffer from having no birefringence or other properties which allow some degree of phase matching and thus the full realisation of the material as frequency converter.
Preferably, the phase matching means are provided in the frequency conversion member to reduce this variation in the refractive indices at the different frequencies.
Alternatively, or in addition to the spatial variation in the refractive index, the phase matching means may be provided for by a periodic modulation in the frequency converting non-linear coefficient along the axis of the input beam or beams.
Preferably, there will be a single input beam which provides two frequency components. This can be achieved by a pulsed laser. Preferably, the pulsed laser will have a pulse width ranging from 10 fs to 10 ps.
Losses due to the frequency conversion member can be reduced if the frequency conversion member is cut at the Brewster""s angle for the polarisation and frequency of the input radiation. Alternatively, the frequency conversion member could be anti-reflection coated to reduce losses at xcfx89vis. Preferably, the frequency conversion member is also a material which has minimum absorption at xcfx89THz due to mechanisms such as phonon absorption etc.
Preferably the two different frequency components are provided by a pulsed laser source. A pulse laser source is desirable because mode matching and beam overlap between the input beams are required to obtain optimum conversion to THz. This problem is circumvented if a single beam provides both frequency components (xcfx89vis1 and xcfx89vis2) Also, the higher fields obtainable by pulsed lasers allow the (progressively smaller) non linearities in the polarisation term to be accessed and the radiation pulse produced by a pulsed laser inherently contains a number of different frequencies making it ideal for difference frequency generation over a broad range.
The radiation source of the second aspect for the present invention is particularly useful in a THz imaging system. Previous THz imaging systems have been bulky because amplifiers external to the laser cavity are required to produce sufficiently large THz powers for applications. Such external amplifiers are bulky, very expensive, and very difficult to operate. Moreover, the addition of such an amplifier results in a reduction of the pulse repetition rate, which lowers the signal to noise ratio associated with the THz image and its quality. These aspects have made the widespread use of THz imaging prohibitive in terms of size, cost, ease of use, quality of images, and image acquisition times. Therefore, an imaging system which uses the source of the second aspect of the present invention provides considerable advantages in that the system is a) more compact, b) less expensive, c) more user-friendly and d) may provide better signal to noise ratios due to the higher pulse repetition rate and higher THz power levels, and e) may provide faster data acquisition times, allowing for collection of THZ images at video frame rates.
Therefore, in a third aspect, the present invention provides an imaging system comprising a radiation source and a detector, the radiation source comprises a frequency conversion member for emitting a beam of radiation in response to a radiation by one or more input beams, the input beam having a frequency different to that of the one or more input beams, the one or more input beam or beams being produced within a lasing cavity and said frequency conversion member being located within said lasing cavity.
It will be appreciated that the radiation source can be configured as described with reference to the first aspect of the present invention. The imaging system basically comprises three main sections, a generation section for generating the imaging radiation (including the THz beam and visible detection beam), an imaging section for imaging a sample and a detection section for detecting the radiation once it has passed through the imaging section. The generation section will be provided by a source in accordance with a first aspect of the present invention.
Preferably, wherever possible mirrors will be used instead of transmission optics to minimise losses associated with transmission optics, i e.
(i) frequency dependent refraction losses and amplitude pattern distortion at dielectric (e.g. air-lens) interfaces
(ii) frequency dependent absorption losses
(iii) diffraction effects and distortions of the field distributions due to power falling on the lens surface at an angle.
Preferably, the THz radiation is directed onto the sample by means of a off axis parabolic (OAP) mirror. In such a mirror, there is a constant phase difference between the incident and reflected beam across the surface of the mirror.
More preferably, an even number of OAP mirrors are used and each adjacent pair are separated by the sum of their focal lengths. In this configuration, the size of the beam waist (minimum beam diameter normal to the beam axis), after reflection from the second mirror in the sequence, is frequency independent. This is also true for the last optical element in a chain providing that there are an even number of optical elements in the chain.
This configuration is particularly advantageous for THz imaging because a THz pulse is made up from a large number of frequency components and it is required to keep the size of the imaging beam constant for all THz frequencies in the pulse. Similar considerations apply for directing the THz radiation, collected from the object which is the subject of the imaging, towards the detection section.
Alternatively or in addition to OAP mirrors, condenser cones may also be used, which may be made of brass or copper, highly polished on the inside and with electro-plating and/or gold/silver evaporated coating. These are preferably located next to the sample which is to be imaged. More preferably within a few wavelength of the sample i.e. 50 xcexcm to 100 xcexcm. The cones preferably have an entrance aperture of diameter about 2 mm and an exit aperture of between 50 xcexcm to 100 xcexcm.
Lenses may also be added, preferably these are chosen from non dispersive materials such as high density polyethylene (EDPE), polytetrafluorethylene (PTFE) and high resistivity Silicon. The materials are preferably non dipersive to avoid temporal broadening of the THz pulse.
Preferably, the imaging system of the third aspect comprises a motorised stage for supporting a sample which is to be imaged. The stage is preferably moveable in two directions orthogonal to the beam axis.
Preferably, the detector used in the imaging system of the third aspect is a non linear crystal and is preferably configured to detect THz radiation using the AC Pockets effect. Here, the detector is configured to rotate the polarisation vector of a first input beam in response to illumination with a THz beam and emit a beam with the polarisation vector rotated.
The first input beam can be thought of as a reference beam, the second beam is the detected beam. Before entering the detector, the polarisation of the reference beam and the detected beam are rotated such that they are parallel to one another and have a component along both the ordinary and extraordinary axes of the modulation region.
The reference beam and the detected beam are preferably linearly polarised before entering the detector. However, one or both of them may also be circularly polarised.
The detector is configured so that the detected beam (if present) rotates the polarisation of the reference beam by an angle. If the detected beam is not present, the reference beam may become slightly elliptically polarised during its passage through the detector crystal. To compensate for this effect, the beam emitted from the detector crystal is preferably passed through an optical correction circuit.
Preferably, the optical correction circuit converts the linearly (or elliptically) polarised beam into a circularly polarised beam.
The emitted linearly (or elliptically) polarised beam is preferably converted into a circularly polarised beam by a variable retardation waveplate e.g. a quarter waveplate. Preferably, this circularly polarised beam is then passed into a polarisation splitting device (such as a Wollaston prism) which separates the circularly polarised radiation into two linear components. If the detected beam is not present, these linear components are equal: If the detected beam is present, these linear components are not equal to one another.
Preferably, the two output beams from the prism are incident on a balanced photodiode assembly, which produces a non-zero output signal if there is a difference in magnitude between the two beams.
Alternatively, the reference beam can be circularly polarised before entering the detector. This can be achieved by putting the variable retardation waveplate in the path of the reference beam before it reaches the detector, and replacing the variable retardation waveplate after the detector with for example, a half waveplate if the variable retardation waveplate is a quater waveplate. The detected beam (if present) changes the circularly polarised reference beam into an elliptically polarised beam. The elliptically polarised beam is then separated into its two components by a Wollaston prism or the like as described previously.
A variation on the prism and balanced photodiode configuration uses two crossed polarizers situated on either side of the detector. The reference beam is passed through the first polarizer, and transmitted through the detection crystal along with the THz beam. If the THz beam is present, the polarisation of the reference beam will be rotated such that the beam has a component in a transmission direction of the second polarizer. If the THz beam is not present, the polarisation of the reference beam is not rotated and hence is blocked by the second polarizer.
The reference beam is preferably produced by the generation section. The radiation carrying the image information is then detected by the detector using the reference beam. Preferably, a control system is provided which provides a time variation between the input beam and the reference beam. The control system may be inserted into either the generation or the reference beam paths.
The control system may comprise a motorised mirror which can be oscillated backwards and forwards along the beam axis in order to increase or decrease the optical path length of the reference beam.
The system may also comprise optics to enlarge the cross sectional area of the reference beam before it enters the detection system. Preferably, the cross sectional area of the reference beam will be larger than that of the imaging radiation, to ensure that the whole of the imaging beam cross-section is detected. The detection section may also comprise a CCD camera with a detection area which is larger or the same size than the area of the reference beam. Thus, the CCD camera reads a 2D image and there is no need to move the sample during imaging.
Alternatively, the control system may comprise a grating pair configured to extend the pulse width of the reference beam. An optical fibre cable can also be configured to extend the pulse width of the reference beam. These cause the different wavelength components of the pulsed reference beam to travel through the detector crystal at very different times.
The detection section may comprises a grating spectrometer, to disperse the wavelengths. The detection section may also comprise a CCD camera to record the spatial distribution from the spectrometer.
Possible materials which posses good non-linear characteristics for either difference frequency generation or detection are GaAs or Si based semiconductors. More preferably, a crystalline structure is used. The following are further examples of possible materials:
NH4H2PO4, ADP, KH2PO4, KH2ASO4, Quartz, AlPO4, ZnO CdS, GaP, BaTiO3, LiTaO3, LiNbO3, Te, Se, ZnTe, ZnSe, Ba2NaNb5O15, AgAsS3, proustite, CdSe, CdGeAs2, AgGaSe2, AgSbS2, ZnS, organic crystals such as DAST (4-N-methylstilbazolium).
In general, non-centrosymmetric crystals are used for second order effects. Third order effects are found in a variety of different crystals with varying strengths.